High frequency range radio transmitters operate throughout a broad range of frequencies. "High frequency" refers to a portion of the frequency spectrum lying between 3 and 30MHz. Air coil inductors are frequently included within high frequency radio systems to establish the operating frequency(s) thereof, and a number of methods have been used to alter the inductance of the coil.
One prior art approach for varying the inductance of a coil has been to mount the coil for rotation about its longitudinal axis. Rods or bars having a contact wheel, or other single projection that engages and follows the coil as it rotates, have been attached to a framework surrounding the coil. Rotation of the coil causes the contact wheel to travel along the length of the coil, thereby providing a continuously variable tap. Typically, the rod or bar is electrically connected to one end of the rotatable coil, so that all of the turns between the connection and the contact wheel are bridged and are thereby nominally shorted out. While the rod-contact wheel variable coil operates generally satisfactorily at the lower end of the high frequency spectrum, parasitic capacitance may develop across the turns of the coil when the coil is set to resonate at a high frequency. Parasitic capacitance within the coil structure itself may result in the formation of an undesirable high frequency, parallel resonance circuit which may act as a trap and absorb significant RF energies intended for other elements or stages of the radio system. Because the coil is mounted for rotation, brushes must be provided to connect to at least one end of the coil. When such a coil is used for transmitting, the possibility of corona discharge or sparking creates ozone which results in tarnishment of the brush contact surfaces. Intermittencies, spurious signals and parasitic oscillations that interfere with the operation of the circuit may result. The use of a rod and contact wheel and supporting framework to vary inductance may also increase the amount of space required to accommodate the variable inductor assembly within the radio system.
A similar prior approach has been to provide a longitudinally positionable bar carrying a rotating contact wheel which effectively rolls over the turns of the coil in order to vary the inductance thereof. This approach had the drawbacks noted for the rotating coil discussed above. In addition, the sliding adjustment bar has not seen widespread use, since adjustment of inductance has typically been accomplished by rotation of a control knob at the control panel of the radio apparatus.
Another prior approach to varying inductance called for connecting wires, commonly known as taps, at various locations throughout the coil and extending the wires to contact switches located at some distance from the coil, as in the Siegrist U.S. Pat. No. 1,679,503. The use of many taps may also result in the formation of undesirable parallel resonance circuits and a lowered overall Q factor for the circuit, thereby reducing the sharpness of the circuit response at the selected frequency. One other prior art approach used to vary inductance includes the use of a motorized, shorting drum structure interior to the coil as in the Hollis U.S. Pat. No. 2,691,141, and the Olson U.S. Pat. No. 3,265,997. The Hollis and Olson drum structures may create parallel resonance problems, and the auxiliary motors and drums also require additional space within the radio system.
Yet another prior approach used to vary inductance placed the coils within a sliding framework as shown in the Marriott U.S. Pat. No. 978,604. The entire coil was moved in and out of the system to vary the inductance.
One additional prior approach to vary inductance is shown in the Benzie et al. U.S. Pat. No. 3,958,196. This approach is somewhat similar to the sliding bar approach noted above. A motor driven pulley apparatus is used to pull a conductive tape through the interior of the coil to progressively short out the windings of the coil. The motor and the pulley system take up space within the radio system and may create parallel resonance problems.
Additional problems with the prior art approaches arose from the use of rods, multiple wires, drums, and auxiliary motors and mounting structures. Such structures are typically conductive, and arcing to surrounding conducting structures can occur, thereby contributing to the creation of parallel circuits between the rods, wires, drums, or motors and the surrounding structures, such as the transmitter enclosure or housing. Additionally, a circuit that uses a single tap or single shorting connection to short out all turns up to the location of the tap is particularly prone to the formation of undesirable parallel resonance circuits.
As can be seen from the discussion of the prior art, an unsolved need exists for an improved variable inductor which overcomes the limitations and drawbacks of the prior art designs.